First:
!addcom !lastdeath Time since last death: No deaths yet.

Then:
!addcom !setdeath !editcom !lastdeath Time since last death: $(eval $+(countup $(time US/Eastern "MMM Do YYYY HH:mm:ss") US/Eastern)).

Edit commands "lastdeath" "setdeath" and "US/Eastern" as needed. See NIGHTBOT TIMEZONES to find your local timezone.

Run !setdeath to reset the timer to count up from the current date and time.

Edited from this code (link expired)**